In such a communication system, messages sent by several transmitters to respective receivers travel over paths sharing a common light guide between an optical multiplexer at an outgoing terminal and an optical demultiplexer at an incoming terminal. Each transmitter is linked with its associated receiver by an individual channel defined by an optical carrier of a particular wavelength differing from those of all the other carriers. These channels, accordingly, occupy predetermined relative positions in the spectrum of wavelengths which are not necessarily related to the geographic locations of the stations. In the ensuing description, the term "adjacent channels" refers to those channels whose carriers lie next to each other in that wavelength spectrum.
The number of channels that can be accommodated by a common light guide is limited by interference phenomena giving rise to cross-talk between adjacent channels. Thus, the carriers must be sufficiently spaced apart to minimize such interference.
Various solutions have already been proposed to overcome the problem of cross-talk. In an article (Paper C7.3) presented by H. Ishio and T. Miki at the IOOC '77 conference held in Tokyo, there has been described a comparison system in which the output signal of a photodetector receiving incoming signals from a channel of a wavelength-division-multiplex system is modified by a corrective electrical signal derived by suitable attenuation and polarity inversion from the optical inter-channel interference.
The output voltage or current of a photodetector oscillates in random fashion about a mean value constituting the useful signal. These oscillations account for a noise component of an amplitude proportional to the actual signal level. Since the noise accompanying an interfering signal is obviously uncorrelated with the noise accompanying the affected signal, the above-described compensation technique cannot reduce noise interference but is in fact likely to intensify same. This problem, accordingly, is specific to optical communication systems using photoelectric signal conversion.
Moreover, the level of the optically interfering signal must be exactly known to enable an effective reduction of cross-talk without further signal degradation. The electrical components needed for signal attenuation and polarity inversion also tend to introduce additional noise due in part to drift and instability phenomena.